Scientists have identified a number of planetary boundaries that must not be passed if we are to maintain the relatively stable environmental conditions of our Holocene era. These are:
- Rate of biodiversity loss – measured by extinction rate
- Nitrogen cycle – measured by amount of N2 removed from the atmosphere for human use
- Phosphorous cycle – measured by quantity of phosphorous flowing in the oceans
- Stratosheric ozone depletion – measured by concentration of ozone
- Ocean acidification – measured by global mean saturation state of aragonite in surface sea water
- Global freshwater use – measured by consumption of freshwater by humans
- Change in land use – measured by percentage of global land cover converted to cropland
- Atmospheric aerosol loading – measured by overall particulate concentration in the atmosphere, on a regional basis
- Chemical Pollution – measured by a number of measurements including the concentration of organic pollutants, plastics, endocrine disrupters etc.
- Climate Change – measured by atmospheric carbon dioxide concentration
Of course all of these boundaries are not separate entities but are interconnected. For instance land clearance for crops decreases habitat for other species and can lead to extinction of species. Also it is important to be aware that while scientists can estimate the boundary levels beyond which the continued stability of the planets ecosystems will be questionable many systems act in a nonlinear fashion and can be very sensitive around threshold levels. These criticisms aside the identification and use of Planetary Boundaries is an useful tool to guide us in making the changes we need to ensure humanity has a future on this planet.
Rockstrom et al. 2009 “A Safe Operating Space for Humanity”. Published in Nature
Owen Gafney 2009. “A Planet on the Edge”. Published in Global Change Issue 74.
Stockholm Resilience Centre www.stockholmresilience.org/planetary-boundaries